This invention relates to an aperture driving apparatus using a voice coil motor, and a projection type image display apparatus using the aperture driving apparatus.
An aperture driving apparatus is used in an imaging apparatus to adjust the amount of light. Recently, such an aperture driving apparatus is not only used in the imaging apparatus, but is also used in a projection type image display apparatus that modulates the light using a light modulating unit (for example, a liquid crystal display panel or a digital micro mirror device) and projects the modulated light onto a screen. In general, the aperture driving apparatus includes an aperture plate made of metal on which a V-shaped cutout is formed, and is configured to partially block the light at the V-shaped cutout to thereby change the amount of light.
Generally, a voice coil motor is used as an actuator of the aperture driving apparatus (see, for example, Patent Document No. 1). In the voice coil motor disclosed in Patent Document No. 1, a coil is mounted to a movable part that swings together with the aperture plate. The coil is wound in a fan-shape so that the winding axis is aligned with the direction of a swing axis of the movable part. Further, a pair of magnets are disposed on both sides of the coil in the direction of the swing axis.
Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2006-284995 (paragraphs 0032-0034 and FIG. 5).
In the above described aperture driving apparatus, there is a problem that the dimension of the fan-shaped coil in the swing direction (i.e., in the direction perpendicular to the swing axis) is large. Therefore, there is proposed a voice coil motor in which a coil is wound in a rectangular shape so that the direction of the winding axis is substantially perpendicular to the swing axis. In such a voice coil motor, a yoke (i.e., a center yoke) made of metal is disposed in the inside of a hollow portion of the coil, a pair of magnets are symmetrically disposed on both sides of the coil in the direction of the swing axis, and a pair of back yokes are disposed on the backsides (i.e., outside) of the respective magnets. The center yoke, the back yokes and the magnets constitute a magnetic path to thereby enhance the efficiency of the voice coil motor.
However, although such a configuration contributes to the reduction of the dimension in the swing direction, the dimension in the direction of the swing axis becomes large since a pair of magnets are disposed on both sides of the coil in the direction of the swing axis.
In order to reduce the dimension of the aperture driving apparatus in the direction of the swing axis, it is considered to dispose only one magnet on only one side of the coil in the direction of the swing axis.
However, with such an asymmetric configuration in which the magnet is disposed on only one side of the coil in the direction of the swing axis, an unnecessary torque may be generated in a direction different from a direction for swinging the movable part. Therefore, there is a problem that vibration may occur when the position of the aperture plate is controlled using a closed-loop control technique.